They do count towards your own mileage but other users on strava will see only mileage from your public activities
No they don't. They do count for the mileage that is shown to your profile but other users do see less mileage without the private activities.
It is really sad that you guys can't implement a...
It is one column on a database table. This is trivial to implement and everyone wants it. Come on Strava
I had this problem since the beginning (well that was last year so I've really never experienced a good Strava route builder). I am just using komoot. Way better than strava for buidling routes